An ultrasonic diagnosis apparatus enables real-time observation of an ultrasonic image of an arbitrary cross section of a subject. Therefore, such an ultrasonic diagnosis apparatus is effectively used to diagnose treatment effects or provide a guide for treatment, through comparison between a reference image captured, for example, before the treatment and an ultrasonic image captured after the treatment (or during the course of the treatment). A diagnostic image such as an ultrasonic image, an X-ray CT image, or an MR image can be used as the reference image. The reference image is generated by extracting, from three-dimensional image data (volume data) of the diagnostic image, two-dimensional image data at a slice position corresponding to the ultrasonic image.
Ultrasonic images are useful, because they are excellent in terms of real time feature, are suitable for observation of a moving organ, and can be captured by use of a simple apparatus. Meanwhile, diagnostic images captured by use of an X-ray CT apparatus or an MRI (magnetic resonance imaging) apparatus are excellent in terms of resolution, and therefore are suitable for use as a reference image. However, such diagnostic images have a characteristic of easily generating an artifact due to motion of an organ or the like.
For example, in the apparatus disclosed in Patent Documents 1 and 2, the position and posture of an ultrasonic probe operated in real time are detected so as to determine the slice position of an ultrasonic image, and a reference image of the same position as the ultrasonic image is extracted from volume data of X-ray CT images or MR images and is displayed on a screen of a monitor or the like.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. Heisei 10-151131    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2002-112998